Sera’s a weird character. You either absolutely adore her or you hate her. And there’s actually an interesting reason behind it. It’s because the character, as written, is actually internally inconsistent. That isn’t poor writing; that’s the character herself. She is, as is many people, flawed. If you happen to be on her good side and happen to select the right responses, she comes across as sweet, funny, brash and loyal. If you happen to be on her bad side and happen to select the wrong responses, she comes across as incredibly insane. The truth isn’t somewhere in between… the truth is both things. Sera is basically the fade. You see what you want to see: it’s a self-fulfilling prophecy.
Consider the “noble” quest. In this personal quest of Sera’s, your speaking with a noble. If you decide to conscript him to the inquisition, kill him or even just to let him go, Sera will still be on friendly terms with you. You might lose approval right away, but once you get back to Skyhold and talk to her, she’ll approve of you again for simply caring what she thinks. For many players, this shows that Sera has a sensitive, reasonable side. Players that go through this quest may have a positive impression of Sera.
But if you question the noble rather than making a decision, Sera goes insane. Not just “insane,” but frothing with madness. She literally kills the noble with her bare hands while you talk to him; there’s nothing you can do to stop it. She continues beating his corpse. This isn’t a guy who was murdering children, by the way; just a typical, run-of-the-mill evil, careless noble. People who happened to want to run their dialogue down completely will be entirely blind-sided by the fact that Sera is, apparently, a psychopath.
And the funny thing is that both these things can be true. Like many Dragon Age: Inquisition characters, Sera is built of layers. She is both quirky and fun and a crazy psychopath. Which side of her you see depends entirely on the way you decide to relate to her in the game. If you only see the quirky and fun side, she’s still a psychopath; you just haven’t tripped up on it yet. If you see the crazy psychopath side, she’s still quirky and fun; you just happened to go a little too deep.
Those on Sera’s good side can see a bit of this in the “roof top cookie” conversation. Sera confesses she hates cookies. She was orphaned early on and taken on by a human woman. She discovers that the human woman couldn’t bake cookies, but instead bought cookies from a baker. The woman was so prideful that she couldn’t admit it, so she instead told Sera that the baker hated elves. Terrible thing to do to a child, yes, but for Sera she made the baker’s life hell and never forgave her adoptive mother, who by all other accounts was extremely kind to her. Sera is just that way: she is just shy of being a sociopath. She can’t forgive the woman for a single, albeit terrible, lie.
Sera’s interesting because all characters don’t need to be paragons of nobility or of evil, of intelligence or stupidity. Sera is smart enough and skilled enough to be an excellent fighter and agent, but she’s not all that clever and she does see the world in terms of black-and-white. She’s flawed, but not flawed in ways that will destroy the world or unleash a terrible evil. She’s just a person. And unlike Leliana, you can’t make Sera any less crazy. She comes on board for her own reasons and she remains a static character throughout. She has no interest in changing.
So whether you actually like Sera is debatable simply because the character is so complex. But the reason why the opinions waver so wildly is because, depending on what you do, you can experience an entirely different Sera than others do.